Common sense guys: how can a soldier march faster on a road than he can on a grass field? Don't let gaming logic (roads = faster movement) override your real world sense.

To a marching army, a road means relatively little. So long as we're not talking about rough, mountainous terrain you can make almost the same time on a road as you can marching through an open field. So a speed bonus makes little sense.

Where roads are useful is for vehicles like carts, wagons, or siege weapons. A wheel works best on a uniform, flat surface, and a road lets them operate ideally with little chance of damage. When moving large amounts of supplies, a road is invaluable.

If anything, a road represents a supply line in which carts and wagons are transporting goods. Protecting a road is therefor critical to the smooth operation of the military or empire they are supporting. The soldiers themselves, however, are not necessarily bound to the roads at all.